It is often the case that members being formed as complicated configurations, and relatively large-size members are manufactured by means of casting; what is more cast products being made of relatively inexpensive cast irons (hereinafter being simply referred to as “cast products”) are used frequently.
In cast iron, carbon (C) in the alloy whose major component is made of iron-carbon exceeds the maximum solid solubility limit in γ iron (e.g., about 2% by mass), and the cast iron is accompanied by eutectoid solidification. Usually, in order to improve the characteristics, such as the mechanical properties, corrosion resistance and heat resistance, various alloying elements are added. Such a cast iron is referred to as an alloy cast iron, and especially those cast irons with great alloying-element amounts are referred to as high-alloy cast irons. These high-alloy cast irons are usually divided into ferritic cast irons and austenitic cast irons roughly depending on the difference between the crystalline structures of their crystallizing bases.
Among them, since the austenitic cast irons are comprised of austenite phase (or γ phase) mainly, not to mention in high-temperature region, but in ordinary-temperature region as well, they are good in terms of heat resistance, oxidation resistance, corrosion resistance, and the like; and are moreover good in terms of ductility, toughness, and so forth. Accordingly, the austenitic cast irons are often used for members that are made use of in harsh environments such as high-temperature atmospheres. For example, speaking of the field of automobiles, turbocharger housings, exhaust manifolds, catalyst cases, and the like, are given. Any of the members are a component part, and so on, respectively, which is exposed to high-temperature exhaust gases, and consequently which is required to exhibit long-term durability.
By the way, various types are available in the austenitic cast irons as well, and the following are representative ones: Niresist, nimol, nicrosilal, monel, minober, nomag, and the like. Moreover, in Japanese Industrial Standards (i.e., JIS), too, 9 types are prescribed for the flake graphitic cast iron (e.g., FCA), and 14 types are prescribed for the spheroidal graphitic cast iron (e.g., FCDA).
In the conventional austenitic cast irons, an austenite phase has been made obtainable even in ordinary-temperature region by having them contain nickel (Ni), namely, an austenite stabilizing element, in a large amount (Ni: from 18 to 36%, for instance). This Ni is expensive considerably compared with iron (Fe), namely, the parent material, and the other alloying elements, and consequently cast products comprising the conventional austenitic cast irons have been highly costly considerably.
For example, a Niresist cast iron being referred to as “D-5S,” which is equivalent to FCDA-NiSiCr3552 according to JIS, is high in terms of austenite-phase stability, and exhibits good oxidation resistance, too, because it includes Ni in a large amount. Moreover, like a Niresist cast iron being referred to as “D-2” that is equivalent to FCDA-NiCr202 according to JIS, austenitic cast irons whose Ni contents are less comparatively have also come to be known publicly. However, the Niresist cast iron that is equivalent to FCDA-NiCr202 is poor in term of oxidation resistance. Consequently, it is unsuitable for a housing for variable nozzle turbocharger (e.g., “VNT™),” being called a variable capacity turbocharger as well), for instance. The VNT™ is a type of turbocharger. It makes the opening areas of a plurality of variable nozzles, which are disposed on the outer side of an exhaust turbine within the housing, variable in compliance with the revolving speeds of an engine, and controls the flow volume of exhaust gases to change the supercharging efficiency, thereby adjusting the revolving speeds of the exhaust turbine. In the VNT™, since the clearance between the housing and the turbine blades affects the flow volume of exhaust gases greatly, the housing's oxidation resistance is important from the viewpoint of securing a given dimension for the clearance. Moreover, there might also be concerns for oxides that have come off from the housing to be bitten between the turbine blades' movable parts so that the turbine blades have come to be unmovable or have been damaged. In addition, since the turbocharger housing as well as exhaust manifolds are component parts that are employed for an exhaust system, the oxides also become a cause of clogging honeycomb supports for exhaust-gas conversion catalyst when they are bulky large-sized ones.
As a cast iron being good in terms of corrosion resistance, Patent Literature No. 1 sets forth a highly-heat-insulating corrosion-resistant cast iron including C in an amount of from 0.8 to 2.0%. Silicon (Si) is added in order to upgrade the heat-insulating property. Moreover, from the viewpoint of corrosion resistance, chromium (Cr) and copper (Cu) are made to be contained. Although Patent Literature No. 1 does not at all refer to the relation between the hardness and composition of cast iron, the cast iron labeled Nos. 1 through 9, which are set forth in the examples, are all unsuitable for processing operations, because any one of them is of high hardness (e.g., about 280 Hv or more by Vickers hardness). Moreover, in Patent Literature No. 1, the cast iron's heat-insulating property is upgraded by making the C content less than those in usual cast irons. In particular, when focusing on the C amount, since the C amounts are from 0.8 to 1.0% in the alloys according to the respective examples being set forth in Patent Literature No. 1, those being disclosed in Patent Literature No. 1 can be referred to as cast steels rather than cast irons.
Moreover, Patent Literature No. 2 discloses an austenitic cast iron whose Si amount is augmented whereas the Ni amount is made much less than that in the foregoing Niresist cast iron. Patent Literature No. 2 discloses that, regarding oxidation resistance, one of the indexes of heat resistance for austenitic cast iron, as the Si amount is augmented, the oxidized weight increase per unit surface area decreases (see FIG. 6 in Patent Literature 2). However, according to studies by the present inventors, when the Si amount becomes excessive, it results in bringing about decline in the elongation of austenitic cast iron, and in deterioration of the machinability. Accordingly, taking the reliability, mass-producibility, and the like, of heat-resistant members comprising austenitic cast irons, it is not realistic at all to simply adjust the Si amount alone in order to enhance the oxidation resistance up to a practically sufficient level.
Hence, the present inventors disclosed an austenitic cast iron in Patent Literature No. 3, austenitic cast iron whose Ni content is less, and which is excellent not only in terms of thermal-fatigue strength, and the like, but also in terms of oxidation resistance. In the austenitic cast iron being set forth in Patent Literature No. 3, the Ni amount becomes a considerably small amount (i.e., the upper limit is 15%) as a whole of the cast iron. From the viewpoint of conventional technical common senses, it seems that no base, in which an austenite phase being stable in ordinary-temperature region makes a major phase, is obtainable. However, they succeeded in obtaining an austenite phase, whose Ni content was even a smaller amount than those conventional ones, by setting the respective contents of C (especially, Cs, a solute carbon content), Si, Cr, manganese (Mn) and Cu, namely, alloying elements other than Ni, so as to fall in appropriate ranges.